Positive electrode active material for non-aqueous electrolyte secondary battery, process for producing the positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery using the positive electrode active material for non-aqueous electrolyte secondary battery

ABSTRACT

The purpose of the present invention is to provide a positive-electrode active material for non-aqueous electrolyte secondary batteries that is capable of achieving both a high capacity and a high output. This positive-electrode active material contains a lithium-nickel composite oxide represented by the general formula: Li b Ni 1-x-y Co x M y O 2  wherein M represents at least one element selected from Al, Ti, Mn and W, b is 0.95≤b≤1.03, x is 0&lt;x≤0.15, y is 0&lt;y≤0.07, and x and y is x+y≤0.16, wherein c-axis length of the lithium-nickel composite oxide is 14.185 angstrom or greater as determined by a Rietveld analysis of X-ray diffraction.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor a non-aqueous electrolyte secondary battery, a process for producingthe positive electrode active material for a non-aqueous electrolytesecondary battery, and a non-aqueous electrolyte secondary battery inwhich the positive electrode active material for a non-aqueouselectrolyte secondary battery is used.

BACKGROUND ART

In recent years, with the rapid spread of a small electronic equipmentsuch as a mobile phone or a notebook-sized personal computer, demand fora non-aqueous electrolyte secondary battery which is used as achargeable and dischargeable power supply has been rapidly increased. Asa positive electrode active material for a non-aqueous electrolytesecondary battery, lithium-nickel composite oxide represented by lithiumnickel dioxide (LiNiO₂) and lithium-manganese composite oxiderepresented by lithium manganese dioxide (LiMnO₂) have been widely usedas well as lithium-cobalt composite oxide represented by lithium cobaltdioxide (LiCoO₂).

However, there are some defects in the lithium cobalt dioxide, such thatthe lithium cobalt dioxide is expensive because its reserve is a littlein the earth, and that the lithium cobalt dioxide contains cobalt whichis unstable in supply and has a highly fluctuating price range as amajor component. Therefore, there have been remarked lithium-nickelcomposite oxide containing relatively inexpensive nickel as a majorcomponent and lithium-manganese composite oxide containing relativelyinexpensive manganese as a major component from the viewpoint ofreducing in costs. The lithium manganese dioxide is superior in thermalstability to lithium cobalt dioxide. However, the lithium manganesedioxide has some problems in practical use in a battery, because itscharge and discharge capacity is much smaller than that of the othermaterials, and its charge and discharge cycle characteristic showinglife of a battery is also much shorter than the other materials. On theother hand, since the lithium nickel dioxide has a charge and dischargecapacity greater than the lithium cobalt dioxide, the lithium nickeldioxide has been expected to be used as a positive electrode activematerial which enables to produce an inexpensive battery having a highenergy density.

The lithium nickel oxide has been usually prepared by mixing a lithiumcompound with a nickel compound such as nickel hydroxide or nickeloxyhydroxide, and calcining the resulting mixture. The form of thelithium nickel oxide is a powder in which primary particles aremono-dispersed or a powder of secondary particles formed by aggregationof primary particles and having spaces between the primary particles.However, both powders have some defects such that the powders areinferior in thermal stability under the condition of charging to thelithium cobalt dioxide. In other words, since pure lithium nickeldioxide has defects in thermal stability, charge and discharge cyclecharacteristics and the like, the lithium nickel dioxide cannot be usedin a practical battery. This is because stability of the crystalstructure of the lithium nickel dioxide is inferior to that of thelithium cobalt dioxide under the condition of charging.

Therefore, in order to stabilize crystal structure under the conditionin which lithium is eliminated, and to obtain lithium-nickel compositeoxide having favorable thermal stability and charge and discharge cyclecharacteristics as a positive electrode active material, there has beengenerally carried out replacement of a part of nickel contained inlithium-nickel composite oxide with other substance. For example, therehas been carried out replacement of a part of nickel with a transitionmetal element such as cobalt, manganese or iron, or a heteroelement suchas aluminum, vanadium or tin (see, for example, Patent Literature 1).

In addition, as a process for improving thermal stability oflithium-nickel composite oxide, there has been developed a process forwashing lithium nickel dioxide with water after calcining (see, forexample, Patent Literatures 2 and 3). When the lithium nickel dioxide iswashed with water after calcining, it is thought that there can beobtained from the lithium nickel dioxide a positive electrode activematerial which has a high volume, and is excellent in thermal stabilityand preservation characteristics under high temperature circumstances inthe case where the positive electrode active material is used in anon-aqueous electrolyte secondary battery.

PRIOR ART LITERATURES Patent Literatures

Patent Literature 1: Japanese Unexamined Patent Publication No. HEI5-242891

Patent Literature 2: Japanese Unexamined Patent Publication No.2003-17054

Patent Literature 3: Japanese Unexamined Patent Publication No.2007-273108

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

When a part of nickel contained in lithium-nickel composite oxide isreplaced with other substance such as other element in a large amount(in other words, under the condition of low content of nickel), althoughthermal stability is improved, capacity of a battery is lowered.

On the other hand, when a part of nickel contained in lithium-nickelcomposite oxide is replaced with an element in a small amount (in otherwords, under the condition of high content of nickel), thermal stabilityis not sufficiently improved.

Moreover, when the content of nickel is increased, there also arises aproblem such that synthesis of the lithium-nickel composite oxidebecomes difficult due to the generation of cation mixing in calcining.

Accordingly, although there have been developed various lithium-nickelcomposite oxides in which a part of nickel is replaced with othersubstance, the lithium-nickel composite oxides have not yet sufficientlyresponded to the requirements for a high capacity and a high output of anon-aqueous electrolyte secondary battery.

When a process for washing lithium nickel dioxide with water is employedafter calcining, there has not yet been obtained lithium nickel dioxidewhich responds to the requirements for a high capacity and a high outputof a non-aqueous electrolyte secondary battery, as well as the above.

As mentioned above, it has not yet been prepared a positive electrodeactive material from lithium-nickel composite oxide which sufficientlyresponds to the requirements for a high capacity and a high output of anon-aqueous electrolyte secondary battery at present. Therefore, it hasbeen desired to develop lithium-nickel composite oxide which satisfiesthe above requirements.

In view of the above circumstances, an object of the present inventionis to provide a positive electrode active material for a non-aqueouselectrolyte secondary battery, which enables to satisfy both a highcapacity and a high output at the same time.

In addition, an object of the present invention is to provide a processfor producing a positive electrode active material for a non-aqueouselectrolyte secondary battery, which enables to easily produce theabove-mentioned positive electrode active material in an industrialscale.

Furthermore, an object of the present invention is to provide anon-aqueous electrolyte secondary battery, in which the above-mentionedpositive electrode active material having a high capacity, a high outputand high safety is employed.

Means for Solving the Problems

(Positive Electrode Active Material for Non-Aqueous ElectrolyteSecondary Battery)

A positive electrode active material for a non-aqueous electrolytesecondary battery according to the first aspect of the present inventionis characterized in that the positive electrode active material includesa lithium-nickel composite oxide represented by the general formula:Li_(b)Ni_(1-x-y)Co_(x)M_(y)O₂wherein M is at least one element selected from the group consisting ofAl, Ti, Mn and W, b satisfies 0.95≤b≤1.03, x satisfies 0<x≤0.15, ysatisfies 0<y≤0.07, and the sum of x and y satisfies x+y≤0.16; and thata length of c-axis of the lithium-nickel composite oxide is 14.185angstrom or more as determined by a Rietveld analysis of X-raydiffraction.

A positive electrode active material for a non-aqueous electrolytesecondary battery according to the second aspect of the presentinvention is characterized in that the above-mentioned lithium-nickelcomposite oxide used in the first aspect of the present invention has aporosity of 0.5 to 4% as determined by the observation of its crosssection with a scanning electron microscope.

A positive electrode active material for a non-aqueous electrolytesecondary battery according to the third aspect of the present inventionis characterized in that the positive electrode active material used inthe first or second aspect of the present invention has an occupancy oflithium of 97 to 99% at 3a site, and an occupancy of metals other thanlithium of 98.5 to 99.5% at 3b site as determined by a Rietveld analysisof X-ray diffraction.

A positive electrode active material for a non-aqueous electrolytesecondary battery according to the fourth aspect of the presentinvention is characterized in that the positive electrode activematerial used in the first, second or third aspect of the presentinvention has a specific surface area of 0.8 to 1.5 m²/g as determinedby a BET method.

A positive electrode active material for a non-aqueous electrolytesecondary battery according to the fifth aspect of the present inventionis characterized in that the positive electrode active material used inany one of the first to fourth aspects of the present invention has amean particle diameter of 8 to 20 μm.

(Process for Producing a Positive Electrode Active Material for aNon-Aqueous Electrolyte Secondary Battery)

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the sixth aspectof the present invention is characterized in that the process includesthe following processes (A) and (B):

(A) mixing a nickel composite hydroxide containing cobalt and at leastone element selected from the group consisting of Al, Ti, Mn and W, anickel composite oxyhydroxide containing cobalt and at least one elementselected from the group consisting of Al, Ti, Mn and W or a nickelcomposite oxide containing cobalt and at least one element selected fromthe group consisting of Al, Ti, Mn and W with a lithium compound, andthereafter calcining the resulting mixture in an oxygen-containingatmosphere at a temperature of 700 to 780° C. so that the length ofc-axis of the lithium-nickel composite oxide is 14.185 angstrom or more,to give a calcined powder of a lithium-nickel composite oxiderepresented by the following general formula (2):Li_(a)Ni_(1-x-y)Co_(x)M_(y)O₂  (2)wherein M is at least one element selected from the group consisting ofAl, Ti, Mn and W; a satisfies 0.98≤a≤1.11; x satisfies 0<x≤0.15; ysatisfies 0<y≤0.07; and the sum of x and y satisfies x+y≤0.16, and(B) mixing the above-mentioned calcined powder of the lithium-nickelcomposite oxide with water so that the amount of the calcined powder ofthe lithium-nickel composite oxide is 700 g to 2000 g per 1 liter ofwater, to form a slurry, washing the calcined powder of thelithium-nickel composite oxide with water under the condition ofmaintaining a temperature of the slurry to 10 to 40° C., and thereafterfiltering and drying the slurry, to give a lithium-nickel compositeoxide powder.

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the seventhaspect of the present invention is characterized in that theabove-mentioned nickel composite oxide is prepared by oxidizing andcalcining at least one of the nickel composite hydroxide and the nickelcomposite oxyhydroxide at a temperature of 500 to 750° C.

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the eighth aspectof the present invention is characterized in that in the sixth aspect ofthe present invention, the above-mentioned nickel composite hydroxide isprepared by adding dropwise an aqueous solution containing nickelsulfate and a metal compound containing cobalt and at least one elementselected from the group consisting of Al, Ti, Mn and W, and an aqueoussolution containing a compound for supplying ammonium ion to a reactionsolution in a reaction vessel being heated, and an aqueous solution ofan alkali metal hydroxide is added dropwise to the reaction solution sothat alkalinity of the reaction solution is maintained during thepreparation of the nickel composite hydroxide.

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the ninth aspectof the present invention is characterized in that in the eighth aspectof the present invention, the above-mentioned nickel composite hydroxideis washed with an aqueous alkaline solution of which pH is controlled to11 to 13 at a liquid temperature of 25° C.

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the tenth aspectof the present invention is characterized in that in the sixth aspect ofthe present invention, the above-mentioned nickel composite oxyhydroxideis prepared by adding an oxidizing agent to the above-mentioned nickelcomposite hydroxide.

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the eleventhaspect of the present invention is characterized in that in any one ofthe sixth to tenth aspects of the present inventions, theabove-mentioned lithium compound is at least one member selected fromthe group consisting of lithium hydroxide, lithium oxyhydroxide, lithiumoxide, lithium carbonate, lithium nitrate and lithium halide.

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the twelfthaspect of the present invention is characterized in that in any one ofthe sixth to eleventh aspects of the present inventions, the nickelcompound is mixed with the lithium compound so that a molar ratio oflithium contained in the lithium compound to all metal elementscontained in the nickel composite oxide is 0.98 to 1.11 in theabove-mentioned process (A).

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the thirteenthaspect of the present invention is characterized in that in any one ofthe sixth to twelfth aspects of the present inventions, the temperatureat washing with water in the process for washing with water iscontrolled to 10 to 40° C. in the above-mentioned process (B).

A process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to the fourteenthaspect of the present invention is characterized in that in any one ofthe sixth to thirteenth aspects of the present inventions, the calcinedpowder is dried after the process for washing with water in anatmosphere not containing a compound including carbon or areduced-pressure atmosphere in the above-mentioned process (B).

(Non-Aqueous Electrolyte Secondary Battery)

A non-aqueous electrolyte secondary battery of the fifteenth aspectaccording to the present invention is characterized in that the positiveelectrode active material for a non-aqueous electrolyte secondarybattery according to any one of the first to fifth aspects of thepresent inventions is used in the non-aqueous electrolyte secondarybattery.

Effects of the Invention

(Positive Electrode Active Material for Non-Aqueous ElectrolyteSecondary Battery)

According to the first aspect of the present invention, when theelectrode active material is used as a positive electrode activematerial for a non-aqueous electrolyte secondary battery, a secondarybattery having a high capacity, a high output and high safety can beobtained.

According to the second aspect of the present invention, an electrolytesolution can be sufficiently permeated to the surface of primaryparticles existing inside the secondary particle, and battery capacityand output characteristics can be furthermore improved.

According to the third aspect of the present invention, lithium-nickelcomposite oxide would not generate cation mixing, its crystallinitybecomes higher, and higher charge and discharge capacity and higheroutput can be achieved.

According to the fourth aspect of the present invention, safety of abattery can be furthermore improved with maintaining a high batterycapacity and high output characteristics.

According to the fifth aspect of the present invention, filling densityin a positive electrode of a battery can be improved with maintaining ahigh battery capacity and high output characteristics.

(Process for Producing a Positive Electrode Active Material for aNon-Aqueous Electrolyte Secondary Battery)

According to the sixth aspect of the present invention, there can beprepared a lithium-nickel composite oxide having a length of c-axis of14.185 angstrom or more which is determined by a Rietveld analysis ofX-ray diffraction and a porosity of 0.5 to 4% even when thelithium-nickel composite oxide contains nickel in a high content.Therefore, when the lithium-nickel composite oxide is used as a positiveelectrode active material, thermal stability and the like can bemaintained, and moreover higher capacity and higher output can beachieved by the facility of releasing and inserting of lithium ions.

According to the seventh aspect of the present invention, when alithium-nickel composite oxide is prepared, the ratio of lithium tometals other than lithium in the lithium-nickel composite oxide can bestabilized. Therefore, a positive electrode active material havinghigher capacity and higher output can be obtained by using thelithium-nickel composite oxide.

According to the eighth aspect of the present invention, since alithium-nickel composite oxide powder having a high bulk density can beprepared, a lithium-nickel composite oxide having a smaller specificsurface area after washing with water can be easily prepared therefrom.

According to the ninth aspect of the present invention, the content ofsulfate radical in the nickel composite hydroxide can be easilycontrolled, and crystallinity and porosity of a lithium-nickel compositeoxide can be easily controlled.

According to the tenth aspect of the present invention, since alithium-nickel composite oxide powder having a high bulk density can beprepared, a lithium-nickel composite oxide having a smaller specificsurface area after washing with water can be easily prepared therefrom.

According to the eleventh aspect of the present invention, since animpurity does not remain after calcining, when the positive electrodeactive material is used in a positive electrode, electric resistance ofthe positive electrode can be lowered.

According to the twelfth aspect of the present invention, sincecrystallinity of calcined powder can be improved, and an excessivelithium compound existing on the surface of a particle can be reduced, abattery having a higher capacity and a higher output can be obtained.

According to the thirteenth aspect of the present invention, sinceelution of lithium in washing with water can be inhibited, a highcapacity, a high output and high safety can be achieved at the sametime.

According to the fourteenth aspect of the present invention, since thecontent of moisture in the lithium-nickel composite oxide can besufficiently reduced, when the lithium-nickel composite oxide is used asa positive electrode active material of a non-aqueous electrolytesecondary battery, generation of gas derived from moisture can beinhibited.

(Non-Aqueous Electrolyte Secondary Battery)

According to the fifteenth aspect of the present invention, since thepositive electrode active material according to any one of the first tofourth aspects of the present inventions is used, cycle characteristicscan be improved with maintaining high capacity and high safety.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a schematic cross-sectional drawing of a coin type battery 1which is used in evaluation of a battery.

FIG. 2 is a schematic explanatory drawing of an equivalent circuit whichis used in a measurement example for evaluation of impedance andanalysis of the impedance.

MODE FOR CARRYING OUT THE INVENTION

The positive electrode active material for a non-aqueous electrolytesecondary battery of the present invention can increase capacity andoutput of a secondary battery, and can improve safety of a secondarybattery when the positive electrode active material is used as apositive electrode active material for a non-aqueous electrolytesecondary battery.

More specifically, the positive electrode active material for anon-aqueous electrolyte secondary battery of the present invention canmaintain thermal stability and the like by controlling the length ofc-axis of the lithium-nickel composite oxide as determined by a Rietveldanalysis of X-ray diffraction even when the positive electrode activematerial has a high content of nickel, and can realize higher capacityand higher output by further controlling the porosity of thelithium-nickel composite oxide particle to increase the easiness forreleasing and inserting of lithium ions.

For example, when the positive electrode active material for anon-aqueous electrolyte secondary battery of the present invention isused as a positive electrode active material for a C2032-type coinbattery, there can be obtained a non-aqueous electrolyte secondarybattery having a high capacity of 205 mAh/g or more and a high output.In other words, the positive electrode active material of the presentinvention is suitable for use as a positive electrode active materialfor a non-aqueous electrolyte secondary battery.

Hereinafter, the positive electrode active material for a non-aqueouselectrolyte secondary battery of the present invention is specificallydescribed.

(Positive Electrode Active Material for a Non-Aqueous ElectrolyteSecondary Battery)

The positive electrode active material for a non-aqueous electrolytesecondary battery of the present invention (hereinafter simply referredto as positive electrode active material of the present invention) is apositive electrode active material which includes a lithium-nickelcomposite oxide represented by the following general formula (1):Li_(b)Ni_(1-x-y)Co_(x)M_(y)O₂  (1)wherein M is at least one element selected from the group consisting ofAl, Ti, Mn and W, b satisfies 0.95≤b≤1.03, x satisfies 0<x≤0.15, ysatisfies 0<y≤0.07, and the sum of x and y satisfies x+y≤0.16, and ischaracterized in that a length of c-axis of the lithium-nickel compositeoxide is 14.185 angstrom or more as determined by a Rietveld analysis ofX-ray diffraction, and that a porosity of the lithium-nickel compositeoxide is 0.5 to 4% as determined by observing the cross section of thelithium-nickel composite oxide with a scanning electron microscope.

(Content of Ni)

The positive electrode active material of the present invention includesa lithium-nickel composite oxide classified into hexagonal system, inwhich “1-x-y” showing the content of nickel (Ni) in the above-mentionedgeneral formula is 0.84 or more.

The upper limit of the content of nickel in the positive electrodeactive material of the present invention is not particularly limited.The higher the content of nickel in the positive electrode activematerial is, the higher the capacity of the positive electrode activematerial becomes. However, when the content of nickel in the positiveelectrode active material is excessively higher, thermal stability ofthe secondary battery becomes insufficient, or cation mixing is apt tobe easily generated in calcining.

To the contrary, when the content of nickel in the positive electrodeactive material is too low, capacity is lowered. When the content ofnickel in the positive electrode active material is less than 0.84,there arise some problems such that the capacity of a battery per volumeof the battery cannot be sufficiently increased even when fillingdensity of the positive electrode is increased.

Therefore, the content of nickel in the positive electrode activematerial of the present invention is preferably 0.84 or more and 0.98 orless, more preferably 0.845 or more and 0.950 or less, furthermorepreferably 0.85 or more and 0.95 or less.

(Content of Co)

The positive electrode active material of the present invention containscobalt (Co). Since the positive electrode active material containscobalt, cycle characteristics of the positive electrode active materialcan be improved.

In the positive electrode active material of the present invention, inaccordance with the increase of the content of cobalt, cyclecharacteristics of the positive electrode active material can be moreimproved. On the other hand, when the content of cobalt exceeds 0.15, itbecomes difficult to increase the capacity of the positive electrodeactive material.

Therefore, in order to improve cycle characteristics of the positiveelectrode active material of the present invention, and to achievehigher capacity, the content of cobalt, which is denoted by “x” in theabove-mentioned general formula is controlled to 0<x≤0.15.

On the other hand, when the content of cobalt is excessively low, sincethere is a possibility such that cycle characteristics of the positiveelectrode active material cannot be sufficiently improved by includingcobalt, the content of cobalt “x” in the positive electrode activematerial of the present invention satisfies preferably 0.03≤x≤0.15, morepreferably 0.05≤x≤0.12.

(Content of Additional Element)

The positive electrode active material of the present invention maycontain an element other than cobalt in order to improve batterycharacteristics.

For example, when at least one element selected from the groupconsisting of Al, Ti, Mn and W is added to the positive electrode activematerial as an additional element which is denoted by “M” in theabove-mentioned general formula, battery characteristics such as thermalstability can be improved.

When the amount of the additional element, which is denoted by “y” inthe above-mentioned general formula, exceeds 0.07, it becomes difficultto achieve higher capacity of the positive electrode active material.When the additional element is not added to the positive electrodeactive material, battery characteristics cannot be improved. Therefore,it is preferred that the amount of the additional element is controlledto 0.01 or more in order to sufficiently improve batterycharacteristics.

Accordingly, in order to improve battery characteristics of the positiveelectrode active material of the present invention, and achieve itshigher capacity, “y” satisfies 0<y≤0.07, preferably 0.01≤y≤0.05.

(Content of Li)

The content of lithium (Li) which is denoted by “b” in theabove-mentioned general formula in the positive electrode activematerial of the present invention is 0.95 or more and 1.03 or less.

When the content of lithium is less than 0.95, battery capacity islowered, and output characteristics are lowered, since a metal elementsuch as Ni is contaminated in a lithium layer contained in a layeredcompound, and inserting and releasing property of Li is lowered. On theother hand, when the content of lithium exceeds 1.03, battery capacityis decreased since Li is contaminated in a metal layer of the layeredcompound.

Therefore, the content of lithium in the positive electrode activematerial of the present invention is 0.95≤b≤1.03, preferably 0.95≤b≤1.01from the viewpoint of maintenance of battery capacity and outputcharacteristics.

(Length of c-Axis)

As mentioned above, the content of nickel in the positive electrodeactive material of the present invention is 0.84 or more, preferably0.98 or more. Therefore, the positive electrode active material containsnickel in a very high content. As mentioned above, in accordance theincrease of the content of nickel, there arises some problems such thatthermal stability is lowered, and the like. Therefore, the content ofnickel is usually controlled so as to be less than 0.84 (the content isgenerally 0.80 or so).

In contrast, according to the positive electrode active material of thepresent invention, since the length of c-axis of the lithium-nickelcomposite oxide as determined by a Rietveld analysis of X-raydiffraction of its crystal is appropriately controlled, a high contentof nickel as mentioned above can be achieved.

In other words, according to the positive electrode active material ofthe present invention, it makes possible to achieve a high content ofnickel by controlling the length of c-axis (hereinafter, simply referredto as length of c-axis) of the lithium-nickel composite oxide to 14.185angstrom or more as determined by a Rietveld analysis of X-raydiffraction of its crystal.

In addition, in case of the lithium-nickel composite oxide having ahexagonal system, such as the positive electrode active material of thepresent invention, the length of c-axis affects releasing property oflithium from a crystal or inserting property of lithium to a crystal.Generally, in accordance with the increase of the length of c-axis, theinterlayer distance between each lithium layer increases. Therefore,releasing property of lithium from a crystal or inserting property oflithium to a crystal is improved. Accordingly, when the lithium-nickelcomposite oxide is used as a positive electrode active material, therecan be obtained a positive electrode active material having a highcapacity and a high output.

On the other hand, when the length of c-axis is shorter, since releasingproperty of lithium from a crystal or inserting property of lithium to acrystal is lowered. Therefore, when the lithium-nickel composite oxideis used as a positive electrode active material, capacity and output ofthe lithium-nickel composite oxide is lowered. In addition, sincecrystallinity is lowered by cation mixing, cycle characteristics andthermal stability of the lithium-nickel composite oxide are lowered. Forexample, when the length of c-axis is less than 14.185 angstrom, sincereleasing property of lithium from a crystal or inserting property oflithium to a crystal is lowered, battery capacity and outputcharacteristics are lowered.

As mentioned above, since the positive electrode active material of thepresent invention has a length of c-axis of 14.185 angstrom or more,releasing property of lithium from a crystal or inserting property oflithium to a crystal of the positive electrode active material isimproved, and the positive electrode active material has a high capacityand a high output.

In other words, since the positive electrode active material of thepresent invention has a length of c-axis of 14.185 angstrom or more,capacity and output of the positive electrode active material can beimproved by the increase of the length of c-axis, as well as a highcapacity of the positive electrode active material based on the increaseof the content of nickel.

Incidentally, the upper limit of length of c-axis is not particularlylimited, and the length of c-axis is preferably 14.205 angstrom or less.In other words, the length of c-axis of the positive electrode activematerial of the present invention is preferably 14.185 angstrom or moreand 14.205 angstrom or less, more preferably 14.185 angstrom or more and14.203 angstrom or less. Controlling the length of c-axis to 14.185angstrom or more and 14.200 angstrom or less is more preferred from theviewpoint of improvement in capacity and thermal stability based on highcrystallinity. When the length of c-axis is too long, there is apossibility such that battery capacity cannot be sufficiently improveddue to the lowering of crystallinity.

(Porosity)

The positive electrode active material of the present invention includessecondary particles formed by aggregation of primary particles of thelithium-nickel composite oxide. It is preferred that a porosity of thesecondary particle is 0.5 to 4% as determined by observing its crosssection with a scanning electron microscope. Since the aggregatedprimary particles are used, releasing and inserting of lithium based onthe contact with an electrolyte solution occur on the surface of theprimary particles. Therefore, the releasing and inserting of lithiumoccur not only on the surface of the secondary particle formed by theaggregation of primary particles, but also in the vicinity of thesurface of the second particle, vacancy in the second particle and onincomplete grain boundary. There is a possibility that fine primaryparticles are slightly contained in the positive electrode activematerial, and most of the particles are formed into secondary particles.When the second particle has only the above-mentioned structure based onaggregation of particles, contact of the particles with an electrolytesolution is insufficient. However, when the above-mentioned porosity ofthe secondary particle is controlled, since the electrolyte solution canbe more sufficiently permeated to the secondary particles and attainedto the surfaces of primary particles, battery capacity and outputcharacteristics can be improved.

When the porosity is less than 0.5%, there is a possibility that theelectrolyte solution cannot be permeated to the secondary particles,which causes insufficient contact of the surfaces of primary particleswith the electrolyte solution. On the other hand, when the porosityexceeds 4%, there is a possibility that thermal stability is lowered bythe excess contact of the surfaces of primary particles with theelectrolyte solution.

Incidentally, the porosity can be determined by observing arbitrarycross section of the above-mentioned secondary particle with a scanningelectron microscope, and analyzing the image of the cross section.

The porosity can be determined, for example, by embedding pluralsecondary particles in a resin or the like; carrying out cross sectionpolisher processing of the resin, or the like so as to enable to observethe cross section of secondary particles; selecting arbitrary 20particles of secondary particles; coloring the vacant part in thesecondary particle black to form a black part, and coloring the compactpart within an outline of the secondary particle white to form a whitepart by means of an image analysis soft such as Win Roof 6.1.1;measuring the total area of the black part and the white part; andcalculating the area ratio in accordance with the formula: [area ofblack part/(area of black part+area of white part)] to determine aporosity.

(Occupancy In Site)

In the positive electrode active material of the present invention, itis preferred that occupancy of lithium (Li) at 3a site is 97 to 99%, andthat occupancy of metals other than lithium at 3b site is 98.5 to 99.5%,as determined by the Rietveld analysis of X-ray diffraction.

When each occupancy at 3a site and 3b site is controlled so as to bewithin the above-mentioned range respectively, there can be achievedhigh crystallinity, high charge and discharge capacity and high outputcharacteristics without the occurrence of cation mixing of thelithium-nickel composite oxide. On the other hand, when each occupancyis without the above-mentioned range, crystallinity is lowered, and alot of metals other than Li are contaminated in 3a site which is to beoriginally occupied by Li. Therefore, migration of Li is remarkablydisturbed by the above-mentioned metals, or an effect of inactivation ofLi which is contaminated in 3b site becomes remarkable, and therebybattery characteristics are sometimes lowered.

(Specific Surface Area)

It is preferred that a specific surface area of the positive electrodeactive material of the present invention is 0.8 to 1.5 m²/g asdetermined by a BET method. Thereby, the contact of the positiveelectrode active material with an electrolyte solution becomessatisfactory, and charge and discharge capacity and outputcharacteristics are improved. On the other hand, when the specificsurface area is smaller than 0.8 m²/g, the contact of the positiveelectrode active material with an electrolyte solution becomesinsufficient, and battery characteristics are sometimes lowered. Inaddition, when the specific surface area exceeds 1.5 m²/g, thermalstability is sometimes lowered.

(Mean Particle Diameter)

It is preferred that the positive electrode active material of thepresent invention has a mean particle diameter of 8 to 20 μm.

When the mean particle diameter of the positive electrode activematerial is smaller than 8 μm, filling density is lowered in the casethat the positive electrode active material is used as a positiveelectrode active material of a battery, and battery capacity per volumeis sometimes lowered.

On the other hand, the mean particle diameter of the positive electrodeactive material exceeds 20 μm, the contact area of the positiveelectrode active material with an electrolyte solution of a battery islowered, and battery capacity and output characteristics are sometimeslowered.

Accordingly, the mean particle diameter of the positive electrode activematerial of the present invention is preferably 8 to 20 μm, morepreferably 8 to 17 μm from the viewpoint of improvement in fillingdensity in the positive electrode with maintaining battery capacity andoutput characteristics.

(Process for Producing a Positive Electrode Active Material for aNon-Aqueous Electrolyte Secondary Battery)

Next, the above-mentioned process for producing a positive electrodeactive material for a non-aqueous electrolyte secondary battery of thepresent invention (hereinafter referred to as process of the presentinvention) is described.

The process for producing a positive electrode active material of thepresent invention is characterized in that the process includes (A) aprocess for calcining and (B) a process for washing with water asmentioned below.

(A) Process for Calcining

First of all, in the process for calcining, a mixture prepared by mixinga nickel composite hydroxide, a nickel composite oxyhydroxide or anickel composite oxide (hereinafter simply referred to as nickelcompound) with a lithium compound is calcined in an oxygen-containingatmosphere such as oxygen atmosphere at a temperature of 700 to 780° C.,to give a calcined powder of a lithium-nickel composite oxiderepresented by the following general formula (2):Li_(a)Ni_(1-x-y)Co_(x)M_(y)O₂  (2)wherein M is at least one element selected from the group consisting ofAl, Ti, Mn and W, a satisfies 0.98≤a≤1.11, x satisfies 0<x≤0.15, ysatisfies 0<y≤0.07, and the sum of x and y satisfies x+y≤0.16.

(B) Process for Washing with Water

In the process for washing with water, the calcined powder of thelithium-nickel composite oxide obtained in the (A) process for calciningis washed with water. More specifically, the calcined powder is mixedwith water so that the amount of the calcined powder is 700 g to 2000 gper 1 L of water to give a slurry, the slurry is washed with water at atemperature of 10 to 40° C., and then the slurry is filtered and dried,to give a powder of a lithium-nickel composite oxide (washed powder withwater).

According to the process of the present invention, the above-mentionedpositive electrode active material for a non-aqueous electrolytesecondary battery of the present invention can be prepared. Morespecifically, there can be prepared a lithium-nickel composite oxidehaving a length of c-axis of 14.185 angstrom or more as determined by aRietveld analysis of X-ray diffraction, even when the lithium-nickelcomposite oxide has a high content of nickel. Therefore, when thelithium-nickel composite oxide obtained in the above is used as apositive electrode active material, the positive electrode activematerial can maintain thermal stability and the like, and moreover ahigh capacity and a high output of the positive electrode activematerial can be achieved since lithium ions can be easily released fromthe positive electrode active material or inserted to the positiveelectrode active material.

Hereinafter, (A) a process for calcining and (B) a process for washingwith water are specifically described.

(A) Process for Calcining

(A) The process for calcining is a process for calcining a mixtureprepared by mixing the nickel compound with the lithium compound, togive a calcined powder of a lithium-nickel composite oxide representedby the above-mentioned general formula (2).

(Nickel Compound)

A nickel compound which is used in the (A) process for calcining is anickel composite hydroxide, a nickel composite oxyhydroxide or a nickelcomposite oxide, each of which contains cobalt, and at least one elementselected from the group consisting of Al, Ti, Mn and W, and preferablyfurther containing 0.1 to 0.4% by weight of sulfate radical.

(Nickel Composite Hydroxide)

A nickel composite hydroxide which is used in the (A) process forcalcining is not particularly limited so long as the nickel compositehydroxide contains the above-mentioned additional element. As the nickelcomposite hydroxide, there can be used a nickel composite hydroxidewhich is prepared by, for example, a process such as a crystallizationmethod, a coprecipitation method or a homogeneous precipitation method.

According to the crystallization method, a nickel composite hydroxidecan be prepared under various conditions. The conditions for preparingthe nickel composite hydroxide are not particularly limited, and it ispreferred that a nickel composite hydroxide is prepared under thefollowing conditions:

More specifically, it is preferred that a nickel composite hydroxide isprepared by adding dropwise an aqueous solution containing nickelsulfate and a metal compound which includes cobalt and at least oneelement selected from the group consisting of Al, Ti, Mn and W, and anaqueous solution containing a compound for supplying ammonium ion to areaction vessel which is heated to 40 to 60° C.

In particular, it is preferred that the nickel composite hydroxide isprepared by adding dropwise an aqueous solution of an alkali metalhydroxide to a reaction vessel as occasion demands so that a reactionsolution becomes alkaline, preferably has a pH of 10 to 14. The elementused as the additional element can be precipitated together with nickel.The nickel composite hydroxide also can be obtained by a process forpreparing a nickel hydroxide with crystallization, and then covering thenickel hydroxide with a metal compound containing an element which isused as the additional element, or a process for impregnating an aqueoussolution containing the metal compound to the nickel hydroxide.

The nickel hydroxide prepared by the above-mentioned crystallizationmethod is a powder having a high bulk density. Therefore, alithium-nickel composite oxide having a small specific surface area canbe easily prepared after washing with water. Accordingly, there can beobtained a nickel composite hydroxide which is suitable for a rawmaterial of a lithium-nickel composite oxide used as a positiveelectrode active material for a non-aqueous electrolyte secondarybattery.

When the nickel hydroxide is crystallized under the condition such thatthe temperature of a reaction solution exceeds 60° C., or that a pH ofthe reaction solution exceeds 14, since a nuclear is preferentiallygenerated in the solution, and growth of a crystal does not proceed,only a fine powder is formed. On the other hand, when the nickelhydroxide is crystallized under the condition such that the temperatureof the reaction solution is lower than 40° C., or that a pH of thereaction solution is lower than 10, the amount of a nuclear generated inthe solution becomes smaller, and a crystal of a particle preferentiallygrows up. In this case, there is a possibility that an obtained nickelhydroxide contains a very large particle which generates a concave andconvex shape when an electrode is produced, and that the reactionsolution contains metal ions of which reaction efficiency is very low ina large residual amount.

Therefore, when the nickel composite hydroxide which is used as a nickelcompound used in the (A) process for calcining is prepared by acrystallization method, it is preferred that the nickel compositehydroxide is prepared under the conditions such that the temperature ofthe reaction solution is maintained to 40 to 60° C., and that thereaction solution is maintained to alkaline, preferably a pH of 10 to14.

(Nickel Composite Oxyhydroxide)

In the (A) process for calcining, a nickel composite oxyhydroxide alsocan be used as the nickel compound. In other words, a nickel compositeoxyhydroxide containing cobalt and at least one element selected fromthe group consisting of Al, Ti, Mn and W can be used as the nickelcompound.

The process for preparing a nickel composite oxyhydroxide is notparticularly limited. It is preferred to use a nickel compositeoxyhydroxide which is prepared by further adding an oxidizing agent suchas sodium hypochlorite or aqueous hydrogen peroxide to theabove-mentioned nickel hydroxide. Since the nickel compositeoxyhydroxide prepared by this method becomes a powder having a high bulkdensity, a lithium-nickel composite oxide having a small specificsurface area after washing with water can be easily prepared from thenickel composite oxyhydroxide. Accordingly, the nickel compositeoxyhydroxide is suitable for a raw material of a lithium-nickelcomposite oxide used in a positive electrode active material for anon-aqueous electrolyte secondary battery.

(Nickel Composite Oxide)

In the (A) process for calcining, a nickel composite oxide also can beused as the nickel compound. In other words, a nickel composite oxidecontaining cobalt and at least one element selected from the groupconsisting of Al, Ti, Mn and W also can be used as the nickel compound.

A process for producing the nickel composite oxide is not particularlylimited. It is preferred that the nickel composite oxide is prepared byoxidizing and calcining the above-mentioned nickel composite hydroxideor the nickel composite oxyhydroxide in an oxygen-containing atmosphereat a temperature of 500 to 750° C., preferably 550 to 700° C. When thenickel composite oxide prepared by this method is used, a compositionratio of Li to metals other than Li in the lithium-nickel compositeoxide can be stabilized in preparing a lithium-nickel composite oxide bycalcining a mixture of the nickel compound and the lithium compound.Thereby, there can be obtained an advantage such that capacity andoutput can be increased when the lithium-nickel composite oxide is usedas a positive electrode active material.

Incidentally, when the temperature for oxidizing and calcining is lowerthan 500° C. in oxidizing and calcining the nickel composite hydroxideor the nickel composite oxyhydroxide, conversion of the nickel compositehydroxide and the like to their oxides sometimes becomes incomplete. Atthat time, it is difficult to stabilize the quality of thelithium-nickel composite oxide which is obtained by using the nickelcomposite oxide in which conversion from the nickel composite hydroxideto the nickel composite oxide is incomplete, and disproportionation ofcomposition during synthesis is apt to occur. In addition, when a nickelcomposite hydroxide and the like are remaining in the nickel compositeoxide obtained after oxidizing and calcining, water vapor is generatedin calcining. Therefore, there sometimes arise some problems such thatthe reaction of the lithium compound with the nickel composite oxide isinhibited, and that crystallinity is lowered.

On the other hand, when the temperature for oxidizing ad calciningexceeds 750° C., crystallinity of an obtained nickel composite oxidebecomes higher, and reactivity of the lithium compound with the nickelcomposite oxide is lowered in a post calcining process. Therefore,crystallinity of a finally obtained lithium-nickel composite oxide islowered, and the length of c-axis sometimes does not become 14.185angstrom or more. Moreover, there are some possibilities such thatprimary particles which compose a nickel composite oxide particlequickly grow up, to cause the formation of gross nickel composite oxideparticles, and that the mean particle diameter of the lithium-nickelcomposite oxide which is obtained by mixing the lithium compound withthe nickel composite oxide and calcining the resulting mixture becomesexcessively large.

Accordingly, when the nickel composite oxide is prepared by oxidizingand calcining the nickel composite hydroxide or the nickel compositeoxyhydroxide in an oxygen-containing atmosphere, the oxidizing andcalcining process is carried out at a temperature of preferably 500 to750° C., more preferably 550 to 700° C.

In addition, the period of time for maintaining the temperature foroxidizing and calcining is preferably 1 to 10 hours, more preferably 2to 6 hours. When the period of time is shorter than 1 hour, conversionto an oxide sometimes becomes incomplete. When the period of timeexceeds 10 hours, crystallinity of the nickel composite oxide sometimesbecomes excessively high.

Incidentally, the atmosphere in oxidizing and calcining can be anoxygen-containing atmosphere, and is preferably the air in considerationof easiness in handling and cost.

The content of sulfate radical (SO₄) in the nickel composite hydroxidewhich is used as a nickel compound is preferably 0.1 to 0.4% by weight,more preferably 0.1 to 0.3% by weight. Thereby, crystallinity of alithium-nickel composite oxide can be easily controlled in the postcalcining process.

In other words, when the content of the sulfate radical in the nickelcomposite hydroxide is controlled to 0.1 to 0.4% by weight, the lengthof c-axis can be easily controlled. In addition, since contraction ofthe secondary particle due to the growth of primary particles incalcining can be appropriately controlled, a porosity of the secondaryparticle can be easily controlled.

However, when the content of the sulfate radical is less than 0.1% byweight, progressing speed of crystallization in calcining becomesexcessively high, and the length of c-axis does not sometimes become14.185 angstrom or more. On the other hand, when the content of thesulfate radical exceeds 0.4% by weight, since growth of primaryparticles is inhibited, a porosity of the secondary particle becomesexcessively large. Also, since crystallization is inhibited, batterycharacteristic are lowered.

In addition, the nickel composite oxyhydroxide and the nickel compositeoxide, which are obtained from the nickel composite hydroxide, containthe sulfate radical in the approximately same content as in the nickelcomposite hydroxide.

Therefore, when the content of the sulfate radical (SO₄) in the nickelcomposite hydroxide is controlled to 0.1 to 0.4% by weight, an activematerial obtained by using a nickel composite oxyhydroxide which isobtained from the nickel composite hydroxide or a nickel composite oxideas a raw material also exhibits the same effects as mentioned above.

It is preferred that the above-mentioned nickel composite hydroxide is anickel composite obtained by a crystallization method. When a sulfatesuch as nickel sulfate is used as a raw material, and the resultingcrystal is sufficiently washed with water several times aftercrystallization, a nickel composite hydroxide containing 0.1 to 0.4% byweight of the sulfate radical is obtained.

Furthermore, it is preferred that washing is carried out by using anaqueous alkaline solution of which pH is controlled to 11 to 13 at aliquid temperature of 25° C. When the pH of the aqueous alkalinesolution is lower than 11, the content of the sulfate radical sometimescannot be lowered to 0.1 to 0.4% by weight. When the pH of the aqueousalkaline solution exceeds 13, not only an effect for decreasing theamount of the sulfate radical is not so improved, but also there is apossibility that a cation would remain in the aqueous alkaline solutionas an impurity.

As the aqueous alkaline solution, an aqueous solution of an alkali metalhydroxide such as sodium hydroxide and an aqueous solution of acarbonate such as sodium carbonate are preferably used. For example,when the aqueous solution of sodium carbonate is used, it is preferredthat washing is carried out by controlling a pH to 11 to 12 at a liquidtemperature of 25° C.

As mentioned above, since the sulfate radical affects crystallization incalcining, the content of the sulfate radical in the nickel compoundwhich is used in calcining can be within the above-mentioned range.Accordingly, the content of the sulfate radical can be adjusted bywashing the nickel composite oxyhydroxide or the nickel composite oxide,preferably by washing with the aqueous alkaline solution.

(Lithium Compound)

The lithium compound which is mixed with the nickel compound is notparticularly limited, and it is preferred that at least one memberselected from the group consisting of lithium hydroxide, lithiumoxyhydroxide, lithium oxide, lithium carbonate, lithium nitrate andlithium halide is used as the lithium compound. When the lithiumcompound is used, there are some advantages such that an impurity wouldnot remain after calcining.

(Molar Ratio of Lithium Compound)

The mixing ratio of the nickel compound to the lithium compound is notparticularly limited. The composition of lithium and metals other thanlithium in the lithium-nickel composite oxide after calcining isapproximately maintained in the mixture obtained by mixing the nickelcompound with the lithium compound. Therefore, it is preferred that themolar ratio of lithium contained in the lithium compound to the totalamount of nickel and other metal elements contained in the nickelcompound is controlled so as to be 0.98 to 1.11.

When the molar ratio is less than 0.98, crystallinity of the resultingcalcined powder is so lowered. Moreover, the molar ratio of lithium tothe metals other than lithium sometimes becomes less than 0.95 in thelithium-nickel composite oxide after the (B) process for washing, andbecomes a factor for causing great lowering in a battery capacity in acharging and discharging cycle.

On the other hand, when the molar ratio exceeds 1.11, since calcining iseasily proceeded, and calcination is apt to be excessively progressed,there is a possibility that the molar ratio exceeds 1.11 in theresulting calcined powder. Moreover, the excessive lithium compoundexists on the surface of the resulting calcined powder in a largeamount, and removal of the excessive lithium compound becomes difficultby washing with water. When the calcined powder thus obtained is used asa positive electrode active material, not only gas is generated from thecalcined powder in a large amount during charging a battery, but alsosince the calcined powder is a powder showing a high pH, the calcinedpowder becomes a factor which arises a problem such that the calcinedpowder reacts with a material such as an organic solvent used inproducing an electrode, to form a gelled slurry. Furthermore, there is apossibility that the molar ratio in the lithium-nickel composite oxideexceeds 1.03 after washing with water, and when the lithium-nickelcomposite oxide is used as a positive electrode active material for abattery, there is a possibility such that its battery capacity islowered, and that internal resistance of the positive electrodeincreases.

Therefore, it is preferred that the mixing ratio of the nickel compoundto the lithium compound is controlled so that the molar ratio of lithiumcontained in the lithium compound to the total amount of nickel andother metal elements contained in the nickel compound is 0.98 to 1.11.

(Calcination)

The mixture obtained by mixing the nickel compound with the lithiumcompound is calcined in an oxygen-containing atmosphere at a temperatureof 700 to 780° C., preferably at a temperature of 730 to 760° C.

When the mixture is calcined at a temperature exceeding 500° C., alithium-nickel composite oxide is generated. However, when thetemperature is lower than 700° C., the resulting crystal does notsufficiently grow up, and its structure becomes unstable. When thelithium-nickel composite oxide thus obtained is used as a positiveelectrode active material, the crystal structure of the positiveelectrode active material is easily destroyed by its phase transitionand the like in charging and discharging.

On the other hand, when the mixture is calcined at a temperatureexceeding 780° C., since cation mixing is easily generated, and alayered structure of the crystal of the lithium-nickel composite oxideis collapsed, there is a possibility that insertion and release oflithium ions become difficult. In addition, the length of c-axis doesnot become 14.185 angstrom or more. Moreover, there are somepossibilities such that the crystal of the lithium-nickel compositeoxide is decomposed to generate nickel oxide, and the like. Furthermore,there are some possibilities such that the particles of thelithium-nickel composite oxide are sintered to form a gross particle ofthe lithium-nickel composite oxide, and that a mean particle diameter ofthe lithium-nickel composite oxide becomes excessively large.

Therefore, it is preferred that the mixture obtained by mixing thenickel compound with the lithium compound is calcined in anoxygen-containing atmosphere at a temperature of 700 to 780° C.,preferably at a temperature of 730 to 760° C. In addition, a period oftime for retaining at a temperature of calcining is preferably 1 to 6hours, more preferably 2 to 4 hours. When the period of time forretaining is shorter than 1 hour, crystallization sometimes becomesinsufficient. When the period of time for retaining exceeds 6 hours,calcining is excessively proceeded, and cation mixing sometimes occurs.

In particular, in order to remove crystallization water from the lithiumcompound, and carry out a reaction homogeneously within a temperaturerange in which the crystal of the lithium-nickel composite oxide growsup, it is especially preferred that calcining is carried out in twosteps of calcining at a temperature of 400 to 600° C. for 1 to 5 hoursand subsequently calcining at a temperature of 700 to 780° C. for 3hours or more.

The above-mentioned calcining can be carried out in an oxidizingatmosphere, preferably an atmosphere of a mixed gas having an oxygenconcentration of 18 to 100% by volume of oxygen and an inert gas, morepreferably an atmosphere of mixed gas having an oxygen concentration of90% by volume or more.

When calcining is carried out in an atmosphere having an oxygenconcentration of 18% by volume or more, that is, an atmosphere of whichoxygen content is higher than the air, a lithium-nickel composite oxidecan be synthesized.

In particular, in order to increase the reactivity between the lithiumcompound and the nickel compound, to give a lithium-nickel compositeoxide excellent in crystallinity, the atmosphere is preferably anatmosphere of a mixed gas having an oxygen concentration of 90% byvolume or more, more preferably an oxygen atmosphere, that is, an oxygenatmosphere having an oxygen concentration of 100%.

Incidentally, the nickel compound is mixed with the lithium compoundprior to calcination. An apparatus and a process for mixing them are notparticularly limited so long as the nickel compound can be homogeneouslymixed with the lithium compound. There can be used as the apparatus, forexample, a dry type mixture such as a V-blender, a mixing-granulationapparatus, and the like.

In addition, an apparatus and a process for calcining a mixture preparedby mixing the nickel compound with the lithium compound are notparticularly limited. As the apparatus, there can be used, for example,a calcining furnace such as an electric furnace, a kiln, a tube furnaceor a pusher furnace, in which its atmosphere can be controlled to anoxygen atmosphere or an atmosphere having an oxygen concentration of 18%by volume or more, such as dried air prepared by removing moisture andcarbon dioxide from the air.

(B) Process for Washing with Water

In the process for washing with water, the calcined powder of thelithium-nickel composite oxide obtained in the (A) process for calciningis washed with water. More specifically, the calcined powder is mixedwith water so that the amount of the calcined powder is 700 g to 2000 gbased on 1 liter of water, to form a slurry, the slurry is washed withwater, and then the washed slurry is filtered and dried, to give apowder of a lithium-nickel composite oxide (washed powder with water).

(Concentration of Slurry)

In the above-mentioned washing with water, the calcined powder of thelithium-nickel composite oxide is mixed with water to form slurry, andthis slurry is stirred to clean the calcined powder of thelithium-nickel composite oxide. At that time, the amount (g) of thecalcined powder of the lithium-nickel composite oxide is controlled soas to be 700 to 2000 g, preferably 700 to 1500 g based on 1 liter ofwater contained in the slurry.

In other words, in accordance with increase of the concentration of theslurry, the amount of the calcined powder of the lithium-nickelcomposite oxide increases in the slurry. When the concentration of theslurry exceeds 2000 g/L, stirring of the slurry becomes difficult sincethe viscosity of the slurry increases. Moreover, since the concentrationof alkali increases in the liquid of the slurry, the dissolution rate ofa deposit adhered to the powder is lowered in accordance with theirequilibrium, and when the deposit is exfoliated from the powder, itbecomes difficult to separate the exfoliated deposit from the powder.

On the other hand, when the concentration of the slurry is lower than700 g/L, since the concentration is excessively low, the amount oflithium eluted from the surface of each particle to the liquidincreases. In particular, the higher the content of the nickel ratio is,the more the eluted amount of lithium is, and the amount of lithiumexisting on the surface of the particle is reduced. At that time,lithium comes to be removed from the crystal lattice of thelithium-nickel composite oxide, and the crystal comes to be easilycollapsed. Therefore, when the lithium-nickel composite oxide thusobtained is used as a positive electrode active material, batterycapacity is lowered.

Accordingly, in view of productivity in an industrial scale, theconcentration of the slurry is controlled to 700 to 2000 g/L, preferably700 to 1500 g/L when washing with water is carried out from theviewpoint of capacity of facilities and workability.

(Temperature of Water)

In the process for washing with water, the temperature in washing withwater, that is, the temperature of the slurry is controlled so as to bepreferably 10 to 40° C. When the temperature in washing with water iscontrolled to the above temperature, an impurity existing on the surfaceof the particle of the lithium-nickel composite oxide can be removed,and the amount of lithium existing on the surface of the particle of thelithium-nickel composite oxide can be reduced to 0.10% by weight or lessbased on the whole of the particles. When the lithium-nickel compositeoxide thus washed is used as a positive electrode active material, itcan be inhibited that gas is generated during maintaining to a hightemperature. Therefore, both a high capacity and a high output, and highsafety can be achieved at the same time.

To the contrary, when the temperature at washing with water is lowerthan 10° C., there are possibilities that the lithium-nickel compositeoxide cannot be sufficiently washed, and that an impurity adhered to thesurface of the particle of the lithium-nickel composite oxide is notsufficiently removed from the surface, and the impurity remains on thesurface in a large amount. When the impurity is remaining on the surfaceof the particle of the lithium-nickel composite oxide, since electricresistance of the surface of the particle of the lithium-nickelcomposite oxide increases, electric resistance of a positive electrodeincreases in the case where the lithium-nickel composite oxide is usedas a positive electrode active material of a battery. Furthermore, thereis a possibility that the specific surface area of the particle of thelithium-nickel composite oxide is excessively lowered. When the specificsurface area is excessively lowered, the reactivity between the particleand an electrolyte solution decreases. When the lithium-nickel compositeoxide thus obtained is used as a positive electrode active material of abattery, it becomes difficult to achieve a high capacity and a highoutput. Moreover, there is a possibility that this impurity containslithium carbonate and lithium hydroxide. In this case, the content oflithium existing on the surface of the particle of the lithium-nickelcomposite oxide exceeds 0.10% by weight, and gas is apt to be generatedduring storing at a high temperature.

On the other hand, when the temperature at washing with water exceeds40° C., the amount of lithium eluted from the particle of thelithium-nickel composite oxide increases, and there is a possibilitythat nickel oxide (NiO) in which Li is removed from its surface ornickel oxyhydroxide (NiOOH) in which Li is substituted with H isgenerated. Since nickel oxide (NiO) and nickel oxyhydroxide (NiOOH) havea high electric resistance, respectively, an electric resistance of thesurface of the particle of the lithium-nickel composite oxide increases,and the content of Li in the particle of the lithium-nickel compositeoxide decreases to cause lowering in capacity.

Accordingly, when the particle of the lithium-nickel composite oxide isused as a positive electrode active material, in order to achieve a highcapacity, a high output and high safety at the same time, change ofwater temperature due to the change of the seasons is suppressed, andthe temperature at washing with water in the process for washing withwater is controlled so as to be preferably 10 to 40° C., more preferably10 to 30° C.

(Period of Time for Washing with Water)

The period of time for washing the calcined powder of the lithium-nickelcomposite oxide with water is not particularly limited, and it isdesired that the period of time is to 10 to 60 minutes or so. When theperiod of time for washing with water is too short, an impurity and alithium compound existing on the surface of the calcined powder cannotbe sufficiently removed from the surface, and remains on the surface. Onthe other hand, when the period of time for washing with water is toolong, washing effect is not so improved, and productivity is lowered.

(Water for Forming Slurry)

There is no particular limitation in water which is used for formingslurry. However, in order to prevent battery performance from loweringdue to the adhesion of an impurity to a positive electrode activematerial, water having an electrical conductivity of less than 10 μS/cmis preferable, and water having an electrical conductivity of 1 μS/cm orless is more preferable.

(Drying Temperature)

A temperature and a method for drying the calcined powder of thelithium-nickel composite oxide after washing with water are notparticularly limited. The drying temperature is preferably 80 to 500°C., more preferably 120 to 250° C.

The reason why the drying temperature is controlled to 80° C. or higheris that the calcined powder of the lithium-nickel composite oxide afterwashing with water is dried promptly in order to prevent the generationof a gradient of the concentration of lithium between the surface andthe interior of the calcined particle of the lithium-nickel compositeoxide.

On the other hand, it is supposed that the surface of the calcinedpowder of the lithium-nickel composite oxide after washing with water isunder the condition such that the ratio of lithium to the metals otherthan lithium is an approximately stoichiometric ratio or that thesurface is almost fully electrically charged due to the desorption oflithium in a little amount. Therefore, when the drying temperatureexceeds 500° C., there is a possibility such that electricalcharacteristics are lowered due to the distortion of the crystalstructure of the powder under the condition being almost fullyelectrically charged.

Accordingly, in order to eliminate anxiety about physical properties andcharacteristics of the calcined powder of the lithium-nickel compositeoxide after washing with water, it is desired that the dryingtemperature is 80 to 500° C., and in consideration of productivity andcost for thermal energy, it is more desired that the drying temperatureis 120 to 250° C.

Incidentally, it is preferred that a process for drying the calcinedpowder of the lithium-nickel composite oxide is carried out by dryingthe powder after filtration at a predetermined temperature with a dryerwhich enables to control the atmosphere in the dryer to an atmospherenot containing a compound having carbon and a compound having sulfur, ora reduced pressure atmosphere. In this case, since the moisture contentin the lithium-nickel composite oxide can be sufficiently reduced, thereis an advantage such that generation of gas derived from the moisturecan be inhibited when the lithium-nickel composite oxide is used as apositive electrode active material for a non-aqueous electrolytesecondary battery.

(Content of Moisture in Powder after Drying)

The content of moisture in the calcined powder of the lithium-nickelcomposite oxide after drying is not particularly limited, and thecontent is preferably 0.2% by weight or lower, more preferably 0.1% byweight or lower. When the content of moisture in the powder exceeds 0.2%by weight, there is a possibility that a lithium compound is generatedon the surface of the powder by absorbing gas containing carbon orsulfur included in the air.

Incidentally, the above-mentioned content of moisture is a value asdetermined by using a Karl Fischer moisture meter at a vaporizationtemperature of 300° C.

(The Others)

In the above-mentioned embodiments, as a process for obtaining acalcined powder of a lithium-nickel composite oxide prior to washingwith water, there has been described a process in which a nickelcompound which is prepared by dissolving or dispersing a metal elementother than lithium by means of a crystallization method is mixed with alithium compound as raw materials, and the resulting mixture iscalcined. However, the process for obtaining a calcined powder of alithium-nickel composite oxide is not particularly limited. As theprocess, there can be cited, for example, a process for carrying out aspray-thermal decomposition of a solution prepared by mixing all ofaqueous solutions containing desired metal elements, and a process forpulverizing and mixing all of compounds containing desired metalelements by means of a mechanical pulverizer such as a ball mill, andthereafter calcining the resulting mixture. It is preferred to use alithium-nickel composite oxide prepared by the above-mentioned processfrom the viewpoint of preparation of a lithium-nickel composite oxidehaving a small specific surface area after washing with water andexcellent thermal stability.

(Non-Aqueous Electrolyte Secondary Battery)

The non-aqueous electrolyte secondary battery of the present inventionis produced by using a positive electrode which is produced by using apositive electrode active material including the above-mentionedlithium-nickel composite oxide, particularly a positive electrode activematerial including a lithium-nickel composite oxide which is prepared bythe above-mentioned process. Since the positive electrode is used in thenon-aqueous electrolyte secondary battery of the present invention, thenon-aqueous electrolyte secondary battery has a high capacity, a highoutput and high safety.

The structure of the non-aqueous electrolyte secondary battery of thepresent invention is explained below.

The non-aqueous electrolyte secondary battery of the present invention(hereinafter simply referred to as secondary battery of the presentinvention) has substantially the same structure as a standardnon-aqueous electrolyte secondary battery, except that the positiveelectrode active material for a non-aqueous electrolyte secondarybattery of the present invention (hereinafter simply referred to aspositive electrode active material of the present invention) is used asa material of a positive electrode.

More specifically, the secondary battery of the present invention has astructure containing a case, and a positive electrode, a negativeelectrode, a non-aqueous electrolyte solution and a separator, which areaccommodated in the case. Furthermore specifically, the secondarybattery of the present invention is formed by laminating a positiveelectrode and a negative electrode through a separator to give anelectrode body, immersing the obtained electrode body in a non-aqueouselectrolyte solution, connecting a positive electrode current collectorof a positive electrode and a negative electrode current collector of anegative electrode with a positive terminal for leading to outside and anegative terminal for leading to outside, respectively, through a leadfor connecting an electrode with a terminal, accommodating them in acase, and sealing the case.

In addition, the structure of the secondary battery of the presentinvention is not limited only to the above-mentioned exemplified onetaand various kinds of outer shapes such as a cylindrical shape and alaminated shape can be employed.

(Positive Electrode)

The positive electrode which is one of the characteristics of thesecondary battery of the present invention is firstly described.

The positive electrode is a sheet-like material. The positive electrodecan be formed by, for example, coating a positive electrode mixturecontaining the positive electrode active material of the presentinvention on the surface of a current collector made of an aluminumfoil, and drying the positive electrode mixture. However, the processfor producing the positive electrode is not particularly limited. Thepositive electrode can be also produced by, for example, supporting apositive electrode mixture containing particles of a positive electrodeactive material and a binder on a belt-like positive electrode coremember (positive electrode current collector).

In addition, the positive electrode is appropriately treated so as tofit a battery to be used. For example, there can be conducted to thepositive electrode a treatment such as a treatment for cutting so as tohave a suitable size corresponding to an objective battery, or atreatment for compressing by means of a roll press and the like in orderto increase an electrode density.

(Positive Electrode Mixture)

The positive electrode mixture can be prepared by mixing the positiveelectrode active material of the present invention in the form of powderwith an electric conductive material and a binder to give a positiveelectrode agent, adding a solvent to the positive electrode agent, andkneading the resulting mixture.

Hereinafter, materials other than the positive electrode activematerial, which are used in the positive electrode mixture, aredescribed.

(Binder)

As a binder which is used in the above-mentioned positive electrodemixture, any of a thermoplastic resin and a thermosetting resin can beused, and the thermoplastic resin is preferred. The above-mentionedthermoplastic resin includes, for example, polyethylene, polypropylene,polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),styrene-butadiene rubber, tetrafluoroethylene-hexafluoropropylenecopolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ethercopolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer,vinylidene fluoride-chlorotrifluoroethylene copolymer,ethylene-tetrafluoroethylene copolymer (ETFE),polychlorotrifluoroethylene (PCTFE), vinylidenefluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylenecopolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE),vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer,vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylenecopolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acidcopolymer, ethylene-methyl acrylate copolymer, ethylene-methylmethacrylate copolymer, and the like. These resins can be used alone, orat least two kinds thereof can be used in combination. In addition, eachof these resins can be a crosslinked resin being crosslinked by Na⁺ ionand the like.

(Electric Conductive Material)

The electric conductive material used in the positive electrode mixturecan be an electric conductive material which is chemically stable in abattery, and the electric conductive material is not particularlylimited. The electric conductive material includes, for example,graphite such as natural graphite (flake graphite and the like) orartificial graphite; carbon black such as acetylene black, Ketjen black,channel black, furnace black, lamp black or thermal black; electricconductive fibers such as carbon fiber and metal fiber; metal powdersuch as aluminum powder; electric conductive whisker such as zinc oxidewhisker or potassium titanate whisker; electric conductive metal oxidesuch as titanium oxide; organic electric conductive materials such aspolyphenylene derivatives; fluorocarbon, and the like. These electricconductive materials can be used alone, or at least two kinds thereofcan be used in combination.

Incidentally, the amount of the electric conductive material which isadded to the positive electrode mixture is not particularly limited, andthe amount is preferably 0.5 to 50% by weight, more preferably 0.5 to30% by weight, furthermore preferably 0.5 to 15% by weight to the powderof a positive-electrode active material contained in the positiveelectrode mixture.

(Solvent)

A solvent is used to dissolve a binder and to disperse a positiveelectrode active material, an electric conductive material and the likein the binder. This solvent is not particularly limited, and there canbe used, for example, an organic solvent such as N-methyl-2-pyrrolidone.

(Positive Electrode Core Material)

A positive electrode core material (positive electrode collector) can bean electronic conductor which is chemically stable in a battery, and isnot particularly limited. The positive electrode core material includes,for example, a foil and a sheet made of a material such as aluminum,stainless steel, nickel, titanium, carbon or an electric conductiveresin. Among them, an aluminum foil, an aluminum alloy foil and the likeare preferred. Moreover, a carbon layer or a titanium layer can beformed on the surface of a foil or a sheet, and an oxide layer can beformed on the surface of a foil or a sheet. Furthermore, a convex andconcave form can be formed on the surface of a foil or a sheet. Inaddition, a net, a punching sheet, a lath, a porous material, a foamedmaterial, a fibrous molded article and the like also can be provided onthe surface of a foil or a sheet.

The thickness of the positive electrode core material is notparticularly limited, and it is preferred that the thickness is, forexample, 1 to 500 μm.

(Materials Other than Positive Electrode)

Next, the materials other than a positive electrode, which are used inthe non-aqueous electrolyte secondary battery of the present invention,are described.

Incidentally, the non-aqueous electrolyte secondary battery of thepresent invention is characterized in that the above-mentioned positiveelectrode active material is used. The materials other than the positiveelectrode can be suitably selected according to the use and requiredcharacteristics, and are not limited to the materials other than thepositive electrode as described below.

(Negative Electrode)

A negative electrode can be one which enables to charge and dischargelithium, and the negative electrode is not particularly limited. As thenegative electrode, there can be used a negative electrode in which anegative electrode mixture containing a negative electrode activematerial and a binder, and an electric conductive material and athickener as optional components, is supported on a negative electrodecore material. The negative electrode can be produced in the same manneras in the positive electrode.

The negative electrode active material can be one which enables toelectrochemically charge and discharge lithium. The negative electrodeactive material includes, for example, graphite, nongraphitizing carbon,lithium alloy, and the like. The lithium alloy is not particularlylimited, and it is preferred that the lithium alloy contains at leastone element selected from the group consisting of silicon, tin,aluminum, zinc and magnesium.

In addition, the mean particle diameter of the negative electrode activematerial is not particularly limited, and it is preferred that the meanparticle diameter is, for example, 1 to 30 μm.

(Binder)

A binder used in the negative electrode mixture can be any of athermoplastic resin and a thermosetting resin. Among the resins,thermoplastic resin is preferred. The thermoplastic resin is notparticularly limited. The thermoplastic resin includes, for example,polyethylene, polypropylene, polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVDF), styrene-butadiene rubber,tetrafluoroethylene-hexafluoropropylene copolymer (FEP),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA),vinylidene fluoride-hexafluoropropylene copolymer, vinylidenefluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylenecopolymer (ETFE), polychlorotrifluoroethylene (PCTFE), vinylidenefluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylenecopolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE),vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer,vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylenecopolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acidcopolymer, ethylene-methyl acrylate copolymer, ethylene-methylmethacrylate copolymer, and the like. These resins can be used alone, orat least two kinds thereof can be used in combination. In addition, eachof these resins can be a crosslinked resin being crosslinked by Nat ionand the like.

(Electric Conductive Material)

The electric conductive material of a negative electrode mixture can bean electric conductive material which is chemically stable in a battery,and the electric conductive material is not particularly limited. Theelectric conductive material includes, for example, graphite such asnatural graphite (flake graphite and the like) or artificial graphite;carbon black such as acetylene black, Ketjen black, channel black,furnace black, lamp black or thermal black; electric conductive fiberssuch as a carbon fiber and a metal fiber; metal powder such as copperpowder or nickel powder; organic electric conductive materials such aspolyphenylene derivatives, and the like. These electric conductivematerials can be used alone, or at least two kinds thereof can be usedin combination.

The amount of this electric conductive material to be added is notparticularly limited. The amount of the electric conductive material ispreferably 1 to 30% by weight, more preferably 1 to 10% by weight to theparticles of a negative electrode active material contained in thenegative electrode material mixture.

(Negative Electrode Core Material)

A negative electrode core material (negative electrode collector) can bean electronic conductor which is chemically stable in a battery, and isnot particularly limited. The negative electrode core material includes,for example, a foil and a sheet made of a material such as stainlesssteel, nickel, copper, titanium, carbon or an electric conductive resin.Among them, copper and copper alloy are preferred. A layer such as acarbon layer, a titanium layer or a nickel layer can be formed on thesurface of a foil or a sheet, and an oxide layer can be formed on thesurface of a foil or a sheet. Moreover, a convex and concave form can beformed on the surface of a foil or a sheet. Furthermore, a net, apunching sheet, a lath, a porous material, a foamed material, a fibrousmolded article and the like can be provided on the surface of a foil ora sheet.

The thickness of the negative electrode core material is also notparticularly limited, and it is preferred that the thickness is, forexample, 1 to 500 μm.

(Non-Aqueous Electrolyte Solution)

It is preferred that the non-aqueous electrolyte solution is a solutionin which a lithium salt is dissolved in a non-aqueous solvent. Thenon-aqueous solvent used in the non-aqueous electrolyte solution is notparticularly limited. The non-aqueous solvent includes, for example,cyclic carbonates such as ethylene carbonate (EC), propylene carbonate(PC), butylene carbonate (BC) and vinylene carbonate (VC); chaincarbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC),ethylmethyl carbonate (EMC) and dipropyl carbonate (DPC); aliphaticcarboxylates such as methyl formate, methyl acetate, methyl propionateand ethyl propionate; lactones such as gamma-butyrolactone andgamma-valerolactone; chain ethers such as 1,2-dimethoxyethane (DME),1,2-diethoxyethane (DEE) and ethoxymethoxy ethane (EME); cyclic etherssuch as tetrahydrofuran and 2-methyltetrahydrofuran; dimethyl sulfoxide,1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane,acetonitrile, propyl nitrile, nitromethane, ethyl monoglyme, phosphoricacid triester, trimethoxymethane, dioxolane derivatives, sulfolane,methyl sulfolane, 1,3-dimethyl-2-imidazolidinone,3-methyl-2-oxazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ethyl ethers, 1,3-propanesultone, anisole,dimethyl sulfoxide, N-methyl-2-pyrrolidone, and the like. These solventscan be used alone, or at least two kinds thereof can be used incombination.

Among them, a mixed solvent of a cyclic carbonate and a chain carbonate,and a mixed solvent of a cyclic carbonate, a chain carbonate and analiphatic carboxylate are particularly preferred.

(Lithium Salt)

The lithium salt being dissolved in a non-aqueous electrolyte solutionincludes, for example, LiClO₄, LiBF₄, LiPF₆, LiAlCl₄, LiSbF₆, LiSCN,LiCl, LiCF₃SO₃, LiCF₃CO₂, Li(CF₃SO₂)₂, LiAsF₆, LiN(CF₃SO₂)₂, LiB₁₀Cl₁₀,lithium lower aliphatic carboxylate, LiCl, LiBr, LiI, chloroboranelithium, lithium tetraphenylborate, lithium imide, and the like. Theselithium salts can be used alone, or at least two kinds thereof can beused in combination. Incidentally, it is preferred that at least LiPF₆is used as the lithium salt.

In addition, the concentration of lithium salt in the non-aqueoussolvent is not particularly limited, and it is preferred that theconcentration is 0.2 to 2 mol/L, more preferably 0.5 to 1.5 mol/L.

(Other Additive)

To the non-aqueous electrolyte solution, various additives other thanthe lithium salt can be added in order to improve charge-dischargecharacteristics of a battery. The additive is not particularly limited.The additive includes, for example, triethyl phosphite, triethanolamine,cyclic ethers, ethylenediamine, n-glyme, pyridine, hexaphosphoric acidtriamide, nitrobenzene derivatives, crown ethers, quaternary ammoniumsalts, ethylene glycol dialkyl ethers, and the like.

(Separator)

A fine separator is intervened between a positive electrode and anegative electrode. This separator is not particularly limited, and itis preferred that the separator is a microporous thin film having a highion permeability, a predetermined mechanical strength and an electricinsulation. In particular, it is preferred that the microporous thinfilm has properties such that holes of the thin film are closed at apredetermined temperature, and that the thin film has a function forincreasing an electric resistance.

The material used in the microporous thin film is not particularlylimited, and there can be used, for example, a polyolefin which isexcellent in organic solvent resistance and has hydrophobicity, such aspolypropylene or polyethylene. In addition, a sheet made of a glassfiber and the like, non-woven fabric, woven fabric and the like can bealso used.

When the separator is a microporous thin film, the diameter of a poreformed in the thin film is not particularly limited, and it preferredthat the diameter of the pore is, for example, 0.01 to 1 μm. Theporosity of the separator is not also particularly limited, and it ispreferred that the porosity is generally 30 to 80%. Moreover, thethickness of the separator is not also particularly limited, and it ispreferred that the thickness is generally 10 to 300 μm.

Furthermore, the separators used in the positive electrode and thenegative electrode can be different from each other. The polymerelectrolyte which includes a non-aqueous electrolyte solution and apolymer substrate for retaining the non-aqueous electrolyte solution canbe used as a separator by unifying into one body with a positiveelectrode or a negative electrode. This polymer substrate is notparticularly limited, as long as the polymer substrate can retain anon-aqueous electrolyte solution. It is preferred that the polymer usedin the polymer substrate is a copolymer of vinylidene fluoride andhexafluoropropylene.

EXAMPLES

The present invention is more specifically described by the followingworking examples of the present invention and comparative examples, butthe present invention is not limited only to those working examples.

Incidentally, in the working examples and the comparative examples, inaccordance with the following methods, metals included in alithium-nickel composite oxide were analyzed, and the length of c-axiswas measured.

(1) Analysis of metals: determined by ICP emission spectrometry.

(2) Measurement of length of c-axis: measured by using a XRDdiffractometer manufactured by PANalytical B. V. under the item numberof X'Pert PRO.

(3) Identification of a particle structure and determination ofporosity:

A positive electrode active material was embedded in a resin, and crosssection polisher processing of the resin was carried out so that thecross section of a particle of the positive electrode active materialcould be observed. The cross section was observed with a scanningelectron microscope manufactured by Hitachi high technologies under theitem number of S-4700 at a magnification of 5000 times. The obtainedimage was processed by using an image analysis soft of WinROOF Ver.6.1.1, and 20 particles or more were arbitrary selected. The vacant areain the secondary particles was colored black to form a black part, andthe compact area within the periphery of the secondary particles wascolored to white to form a white part. The total area of the black partand the white part was measured, and the area ratio of [area of blackpart/(area of black part+area of white part)] was calculated todetermine porosity.

(4) Specific surface area: determined by using a specific surface areameasuring apparatus commercially available from Yuasa Ionics Inc. underthe trade name of Multisorb 16 in accordance with a single point BETmethod using adsorption of nitrogen.

(5) Evaluation of battery characteristics:

(Process for Producing Secondary Battery for Evaluating BatteryPerformance)

When battery performance of a non-aqueous electrolyte secondary battery,in which a lithium-nickel composite oxide according to the presentinvention was used as a positive electrode active material, wasevaluated, a 2032 coin type battery as shown in FIG. 1 (hereinafterreferred to as coin type battery 1) was used.

As shown in FIG. 1, the coin type battery 1 is composed of a case 2 andan electrode 3 which is accommodated in the case 2.

The case 2 has a positive electrode can 2 a which is hollow and has anopening at one end, and a negative electrode can 2 b which is arrangedat the opening of the positive electrode can 2 a. When the negativeelectrode can 2 b is placed on the opening of the positive electrode can2 a, a space for accommodating an electrode 3 is formed between thenegative electrode can 2 b and the positive electrode can 2 a.

The electrode 3 includes a positive electrode (electrode for evaluation)3 a, a separator 3 c and a negative electrode (lithium metal negativeelectrode) 3 b, and these constituents are laminated in this order. Thepositive electrode 3 a and the negative electrode 3 b are accommodatedin the case 2 so that the positive electrode 3 a is contacted with theinner surface of the positive electrode can 2 a, and that the negativeelectrode 3 b is contacted with the inner surface of the negativeelectrode can 2 b.

Incidentally, the case 2 is equipped with a gasket 2 c. The positiveelectrode can 2 a and the negative electrode 2 b are maintained so thatthe positive electrode can 2 a is not contacted with the negativeelectrode 2 b through the gasket 2 c, and that a relative movement ofthe positive electrode can 2 a and the negative electrode 2 b are fixedby the gasket 2 c. Also, since the gasket 2 c seals the space betweenthe positive electrode can 2 a and the negative electrode 2 b, thegasket 2 c has a function for airtightly and liquid-tightly blocking thespace between the inside of the case 2 and its outside.

The above-mentioned coin type battery 1 was produced by the followingmethods:

At first, 5 parts by weight of acetylene black and 5 parts by weight ofpolyvinylidene fluoride were mixed with 90 parts by weight of a powderof a positive electrode active material, and n-methyl pyrrolidone isadded to the resulting mixture, to give a paste. An aluminum foil havinga thickness of 20 μm was coated with this paste, so that the amount ofthe positive electrode active material after drying was 0.05 g/cm².

Thereafter, the aluminum foil which was coated with the paste was driedat 120° C. under reduced pressure, and then the aluminum foil waspunched into a disc having a diameter of 1 cm, to give a positiveelectrode 3 a.

The above-mentioned coin type battery 1 was produced by using thispositive electrode 3 a, the negative electrode 3 b, the separator 3 cand an electrolyte solution in a glove box having argon gas atmosphereof which dew point was controlled to −80° C.

Incidentally, as the negative electrode 3 b, a discoidal plate made oflithium metal having a diameter of 15 mm was used.

As the separator 3 c, a porous polyethylene film having a thickness of20 μm was used.

As the electrolyte solution, a mixed solution of ethylene carbonate (EC)and diethyl carbonate (DEC) being mixed in an equal ratio, containing 1M of LiClO₄ as a supporting electrolyte (manufactured by Ube Industries,Ltd.) was used.

The battery characteristics of the coin type battery produced by theabove-mentioned method were evaluated. As the battery characteristics,initial electric discharge capacity and positive electrode reactionresistance were determined.

The initial discharge capacity was determined by the following method:

At first, a coin type battery 1 was produced, and the coin type battery1 was allowed to stand for about 24 hours. After the open circuitvoltage OCV (Open Circuit Voltage) of the coin type battery 1 wasstabilized, the positive electrode of the coin type battery 1 wascharged up to a cutoff voltage of 4.3 V at a current density of 0.1mA/cm². After 1 hour pauses, the coin type battery 1 was discharged to acutoff voltage of 3.0 V. The coin type battery 1 was discharged to acutoff voltage of 3.0 V, and the capacity of the coin type battery 1 atthis point was regarded as an initial discharge capacity.

The positive electrode reaction resistance was calculated by thefollowing method:

First of all, the coin type battery was charged to a charging voltage of4.1 V. An AC impedance of the coin type battery was determined by an ACimpedance method with a frequency response analyzer and apotentiogalvanostat (manufactured by Solartron ISA under the item numberof 1255B). As a result, a Nyquist plot was obtained as shown in FIG. 2.This Nyquist plot showed a sum of characteristic curves indicating asolution resistance, a negative electrode resistance and its capacity,and electric resistance of a positive electrode and its capacity.Therefore, a fitting calculation was carried out by using an equivalentcircuit based on the Nyquist plot, to obtain a value of the positiveelectrode reaction resistance. Incidentally, as the electric resistanceof a positive electrode, a relative value when the electric resistanceof a positive electrode obtained in Example 1 was regarded as 1.00 wasused.

Example 1

First of all, a known crystallization method was used. While maintaininga pH of the reaction solution to 13.0 at a liquid temperature of 25° C.with 20% by weight aqueous solution of sodium hydroxide, a mixed aqueoussolution of nickel sulfate and cobalt sulfate, an aqueous solution ofsodium aluminate and 25% aqueous ammonia were added to the reactionsolution, and the resulting solution was collected by overflowing. Thecollected solution was washed with 45 g/L aqueous solution of sodiumhydroxide having a pH of 12.5 at a liquid temperature of 25° C., thenwashed with water, and dried, to give a nickel composite hydroxide(neutralizing crystallization method).

This nickel composite hydroxide was composed of spherical secondaryparticles which were formed by the aggregation of plural primaryparticles having a diameter of 1 μm or less. When the nickel compositehydroxide was analyzed by an ICP method, it was confirmed that thenickel composite hydroxide was a nickel composite hydroxide in which themolar ratio of Ni:Co:Al was 85:12:3.

The mean particle diameter on the basis of volume (MV) of this nickelcomposite hydroxide was 15 μm as determined by a laser diffractionscattering method. In addition, the quantitative analysis of sulfur wascarried out by an ICP emission spectrometry, and the content of sulfateradical was determined by regarding that sulfur is completely oxidizedto sulfate radical (SO₄). As a result, the content of the sulfateradical was 0.28% by weight. The content of the sulfate radical in thenickel composite hydroxide is shown in Table 1.

This nickel composite hydroxide was oxidized and calcined at 600° C. inthe air, to give nickel composite oxide. Thereafter, the nickelcomposite hydroxide and lithium hydroxide monohydrate (manufactured byWako Pure Chemical Industries, Ltd.) were weighed, and the nickelcomposite hydroxide was mixed with the lithium hydroxide monohydrate sothat the molar ratio of Ni:Co:Al:Li was 0.85:0.12:0.03:1.03, to give alithium-containing mixture.

The resulting lithium-containing mixture was provisionally calcined at500° C. for 3 hours in an oxygen-containing atmosphere by using anelectric furnace, then the mixture was maintained at 750° C. for 3hours, and calcining was carried out by controlling the period of timefrom the initiation of increasing the temperature to the end ofmaintaining the temperature to 20 hours. Thereafter, the mixture wascooled to room temperature in the electric furnace, and pulverized togive a spherical calcined powder made by the aggregation of primaryparticles.

To the obtained calcined powders, purified water having a temperature of20° C. was added, to form slurry containing the calcined powder in anamount of 1000 g per 1 liter the purified water. This slurry was stirredfor 50 minutes for washing with water.

Thereafter, powder was collected by filtering the slurry, and the powderwas allowed to stand for 10 hours in a vacuum drier which was heated to150° C., to give a lithium-nickel composite oxide. The mean particlediameter MV of the resulting lithium-nickel composite oxide was 15 μm,which was substantially the same as that of the raw material, the nickelcomposite hydroxide.

The formulation of the obtained lithium-nickel composite oxide wasanalyzed, and the length of c-axis of the lithium-nickel composite oxidewas determined. The occupancy of lithium (Li) at 3a site was 98.8% asdetermined by a Rietveld analysis of X-ray diffraction, and theoccupancy of metals other than lithium at 3b site was 99.3% asdetermined by the Rietveld analysis.

In addition, a coin type battery was produced by using the powder of thelithium-nickel composite oxide obtained above in accordance with themethod as mentioned above. The initial discharge capacity and thepositive electrode reaction resistance of the produced coin type batterywere determined.

The results are shown in Table 2.

Example 2

A positive electrode active material was prepared and evaluated in thesame manner as in Example 1, except that crystallization of a nickelcomposite hydroxide was carried out so that the molar ratio of nickel,cobalt and aluminum was 90:7:3, to give a nickel composite hydroxide.

The results are shown in Table 1 and Table 2, respectively.

Example 3

A positive electrode active material was prepared and evaluated in thesame manner as in Example 1, except that crystallization of a nickelcomposite hydroxide was carried out so that the molar ratio of nickel,cobalt and aluminum was 94:3:3, to give a nickel composite hydroxide.

The results are shown in Table 1 and Table 2, respectively.

Example 4

A positive electrode active material was prepared and evaluated in thesame manner as in Example 1, except that crystallization of a nickelcomposite hydroxide was carried out so that the molar ratio of nickel,cobalt and aluminum was 90:7:3, to give a nickel composite hydroxide,and that 10 g/L aqueous sodium hydroxide solution having a pH of 11.0 ata liquid temperature of 25° C. was used in washing the nickel compositehydroxide after its collection by overflowing.

The results are shown in Table 1 and Table 2, respectively.

Example 5

A positive electrode active material was prepared and evaluated in thesame manner as in Example 1, except that crystallization of a nickelcomposite hydroxide was carried out so that the molar ratio of nickel,cobalt and aluminum was 90:7:3, to give a nickel composite hydroxide,and that the nickel composite hydroxide after its collection byoverflowing was washed with water only one time.

The results are shown in Table 1 and Table 2, respectively.

Comparative Example 1

A positive electrode active material was prepared and evaluated in thesame manner as in Example 1, except that crystallization of a nickelcomposite hydroxide was carried out so that the molar ratio of nickel,cobalt and aluminum was 82:15:3, to give a nickel composite hydroxide.

The results are shown in Table 1 and Table 2, respectively.

Comparative Example 2

A positive electrode active material was prepared and evaluated in thesame manner as in Example 1, except that crystallization of a nickelcomposite hydroxide was carried out so that the molar ratio of nickel,cobalt and aluminum was 90:7:3, to give a nickel composite hydroxide,and that 65 g/L aqueous sodium hydroxide solution having a pH of 13.5 ata liquid temperature of 25° C. was used in washing the nickel compositehydroxide after its collection by overflowing.

The results are shown in Table 1 and Table 2, respectively.

(Evaluation)

In Examples 1 to 5, the lithium-nickel composite oxides were prepared bythe process according to the present invention. Therefore, as shown inTable 1, it can be confirmed that the length of c-axis can be controlledto 14.185 angstrom or more, and that high battery capacity and lowelectric resistance of a positive electrode (high outputcharacteristics) are achieved. In particular, in Examples 1 to 4, it canbe confirmed that crystallization is sufficiently progressed, and thathigher battery capacity and lower electric resistance of a positiveelectrode (high output characteristics) are achieved, since the porosityis within a range of 0.5 to 4%.

On the other hand, in Comparative Example 1, since the content of nickelis lower, and the length of c-axis is less than 14.185 angstrom, itsbattery capacity is lower than that of each Example, and electricresistance of a positive electrode is higher than that of each Example.

As to Comparative Example 2, since the content of the sulfate radical inthe nickel hydroxide was lower, and crystallization is excessivelypreceded, the length of c-axis is less than 14.185 angstrom, itsporosity is higher than that of each Example, its battery capacity islower than that of each Example, and its electric resistance of apositive electrode is higher than that of each Example.

From the results as mentioned above, when a lithium-nickel compositeoxide is prepared by the process according to the present invention, anda non-aqueous electrolyte secondary battery is produced by using thislithium-nickel composite oxide as a positive electrode active material,it can be confirmed that a battery having high initial dischargecapacity can be produced.

TABLE 1 Conditions for washing with water Washing Content of Temper-Temper- Concen- with sulfate ature at ature tration alkali radicalcalcining of water of slurry (pH) (wt %) (° C.) (° C.) (g/L) Example 112.5 0.28 750 20 1000 Example 2 12.5 0.26 750 20 1000 Example 3 12.50.28 750 20 1000 Example 4 11.0 0.12 750 20 1000 Example 5 — 0.84 750 201000 Comp. Ex. 1 12.5 0.27 750 20 1000 Comp. Ex. 2 13.5 0.08 750 20 1000

TABLE 2 Composition Mean particle Length Specific Discharge Electric(molar ratio of diameter of c-axis Porosity surface area capacityresistance of a Ni:Co:Al) Li/M (μm) (Å) (%) (m²/g) (mAh/g) positiveelectrode Example 1 85:12:3 0.96 15 14.185 3.4 1.25 208 1.00 Example 290:7:3 0.95 15 14.190 3.4 1.20 210 0.95 Example 3 94:3:3 0.96 14 14.2003.2 1.15 211 0.91 Example 4 90:7:3 0.96 15 14.191 0.8 0.91 209 0.98Example 5 90:7:3 0.94 15 14.205 0.68 1.61 201 1.15 Comp. Ex. 1 82:15:30.97 16 14.180 3.8 1.43 200 1.25 Comp. Ex. 2 90:7:3 0.97 15 14.183 0.310.70 193 1.16

INDUSTRIAL APPLICABILITY

The non-aqueous electrolyte secondary battery of the present inventionhas a high capacity and high safety. Therefore, the non-aqueouselectrolyte secondary battery of the present invention can beparticularly suitably used as a secondary battery capable of chargingand discharging, which can be used in small portable electronicequipments such as a notebook-sized personal computer and a mobilephone.

EXPLANATION OF REFERENTIAL NUMBERS

-   -   1 Coin type battery    -   2 Case    -   2 a Positive electrode can    -   2 b Negative electrode can    -   2 c Gasket    -   3 Electrode    -   3 a Positive electrode    -   3 b Negative electrode    -   3 c Separator

The invention claimed is:
 1. A process for producing a positiveelectrode active material for a non-aqueous electrolyte secondarybattery, comprising the following processes (A) and (B): (A) mixing anickel composite hydroxide comprising cobalt and at least one elementselected from the group consisting of Al, Ti, Mn and W, a nickelcomposite oxyhydroxide comprising cobalt and at least one elementselected from the group consisting of Al, Ti, Mn and W, or a nickelcomposite oxide comprising cobalt and at least one element selected fromthe group consisting of Al, Ti, Mn and W with a lithium compound, andthereafter calcining the resulting mixture in an oxygen-containingatmosphere at a temperature of 700 to 780° C. so that the length ofc-axis of the lithium-nickel composite oxide is 14.185 angstrom or more,to give a calcined powder of a lithium-nickel composite oxiderepresented by the following general formula (2):Li_(a)Ni_(1-x-y)Co_(x)M_(y)O₂  (2) wherein M is at least one elementselected from the group consisting of Al, Ti, Mn and W; a satisfies0.98≤a≤1.11; x satisfies 0<x≤0.15; y satisfies 0<y≤0.07; and the sum ofx and y satisfies x+y≤0.16, and (B) mixing said calcined powder of thelithium-nickel composite oxide with water so that the amount of thecalcined powder of the lithium-nickel composite oxide is 700 g to 2000 gper 1 liter of water to form a slurry, washing the calcined powder ofthe lithium-nickel composite oxide with water under the condition ofmaintaining a temperature of the slurry to 10 to 40° C., and thereafterfiltering and drying the slurry, to give a lithium-nickel compositeoxide powder.
 2. The process for producing a positive electrode activematerial for a non-aqueous electrolyte secondary battery according toclaim 1, wherein the nickel composite oxide is prepared by oxidizing andcalcining at least one of the nickel composite hydroxide and the nickelcomposite oxyhydroxide at a temperature of 500 to 750° C.
 3. The processfor producing a positive electrode active material for a non-aqueouselectrolyte secondary battery according to claim 1, wherein said nickelcomposite hydroxide is prepared by adding dropwise an aqueous solutioncomprising nickel sulfate and a metal compound comprising cobalt and atleast one element selected from the group consisting of Al, Ti, Mn andW, and an aqueous solution comprising a compound for supplying ammoniumion to a reaction solution in a reaction vessel being heated, andwherein an aqueous solution of an alkali metal hydroxide is addeddropwise to the reaction solution so that alkalinity of the reactionsolution is maintained during the preparation of the nickel compositehydroxide.
 4. The process for producing a positive electrode activematerial for a non-aqueous electrolyte secondary battery according toclaim 3, wherein said nickel composite hydroxide is washed with anaqueous alkaline solution of which pH is controlled to 11 to 13 at aliquid temperature of 25° C.
 5. The process for producing a positiveelectrode active material for a non-aqueous electrolyte secondarybattery according to claim 1, wherein the nickel composite oxyhydroxideis prepared by adding an oxidizing agent to said nickel compositehydroxide.
 6. The process for producing a positive electrode activematerial for a non-aqueous electrolyte secondary battery according toclaim 1, wherein said lithium compound is at least one member selectedfrom the group consisting of lithium hydroxide, lithium oxyhydroxide,lithium oxide, lithium carbonate, lithium nitrate and lithium halide. 7.The process for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to claim 1, whereinthe nickel compound is mixed with the lithium compound so that a molarratio of lithium contained in the lithium compound to all metal elementscontained in the nickel composite oxide is 0.98 to 1.11 in the process(A).
 8. The process for producing a positive electrode active materialfor a non-aqueous electrolyte secondary battery according to claim 1,wherein the temperature at washing with water in the process for washingwith water is controlled to 10 to 40° C. in the process (B).
 9. Theprocess for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to claim 1, whereinthe calcined powder after washing with water is dried in an atmospherenot containing a compound comprising carbon or in a reduced-pressureatmosphere in the process (B).
 10. The process for producing a positiveelectrode active material for a non-aqueous electrolyte secondarybattery according to claim 2, wherein said lithium compound is at leastone member selected from the group consisting of lithium hydroxide,lithium oxyhydroxide, lithium oxide, lithium carbonate, lithium nitrateand lithium halide.
 11. The process for producing a positive electrodeactive material for a non-aqueous electrolyte secondary batteryaccording to claim 3, wherein said lithium compound is at least onemember selected from the group consisting of lithium hydroxide, lithiumoxyhydroxide, lithium oxide, lithium carbonate, lithium nitrate andlithium halide.
 12. The process for producing a positive electrodeactive material for a non-aqueous electrolyte secondary batteryaccording to claim 4, wherein said lithium compound is at least onemember selected from the group consisting of lithium hydroxide, lithiumoxyhydroxide, lithium oxide, lithium carbonate, lithium nitrate andlithium halide.
 13. The process for producing a positive electrodeactive material for a non-aqueous electrolyte secondary batteryaccording to claim 5, wherein said lithium compound is at least onemember selected from the group consisting of lithium hydroxide, lithiumoxyhydroxide, lithium oxide, lithium carbonate, lithium nitrate andlithium halide.
 14. The process for producing a positive electrodeactive material for a non-aqueous electrolyte secondary batteryaccording to claim 2, wherein the nickel compound is mixed with thelithium compound so that a molar ratio of lithium contained in thelithium compound to all metal elements contained in the nickel compositeoxide is 0.98 to 1.11 in the process (A).
 15. The process for producinga positive electrode active material for a non-aqueous electrolytesecondary battery according to claim 3, wherein the nickel compound ismixed with the lithium compound so that a molar ratio of lithiumcontained in the lithium compound to all metal elements contained in thenickel composite oxide is 0.98 to 1.11 in the process (A).
 16. Theprocess for producing a positive electrode active material for anon-aqueous electrolyte secondary battery according to claim 4, whereinthe nickel compound is mixed with the lithium compound so that a molarratio of lithium contained in the lithium compound to all metal elementscontained in the nickel composite oxide is 0.98 to 1.11 in the process(A).
 17. The process for producing a positive electrode active materialfor a non-aqueous electrolyte secondary battery according to claim 5,wherein the nickel compound is mixed with the lithium compound so that amolar ratio of lithium contained in the lithium compound to all metalelements contained in the nickel composite oxide is 0.98 to 1.11 in theprocess (A).
 18. The process for producing a positive electrode activematerial for a non-aqueous electrolyte secondary battery according toclaim 6, wherein the nickel compound is mixed with the lithium compoundso that a molar ratio of lithium contained in the lithium compound toall metal elements contained in the nickel composite oxide is 0.98 to1.11 in the process (A).